Polytetrafluoroethylene laminated article

ABSTRACT

To provide a PTFE laminated article having a sufficient adhesive strength and a PTFE backing sheet having a low permeability, in which a specific weight of a PTFE sheet can be maintained high. A specific weight of the PTFE sheet is maintained at 2.175 or more by carrying out thermo-fusing of the PTFE sheet and a film or sheet of heat-meltable resin such as PFA after interposing a layer of PTFE fine particles between them or by carrying out treatment for preventing a rapid temperature lowering after the thermo-fusing without interposing the layer of PTFE fine particles.

TECHNICAL FIELD

The present invention relates to a polytetrafluoroethylene (PTFE)laminated article having a low permeability to a chemical solution,namely a reduced permeability to a chemical solution. The laminatedarticle is useful for a backing sheet to be used for a chemical solutionstorage vessel or tank, lining of piping, and the like.

BACKGROUND ART

Various chemical solutions are widely used as a starting solution or adetergent not only in production of semi-conductors but also in variouschemical plants. Among the chemical solutions, there are those beinghigh in reactivity or being corrosive. For vessels and pipes for storingor transporting such chemical solutions, usually a laminated articlecalled a backing sheet is lined on inner walls thereof. A surface of thebacking sheet contacting a chemical solution must be made of a materialbeing excellent in chemical resistance, and usually a sheet offluorine-containing resin being excellent in chemical resistance,particularly a sheet of PTFE is used.

A typical backing sheet which has been used so far is one made byadhering a heat resistant woven fabric of glass fiber or carbon fiber toa PTFE sheet. However since it is difficult to allow the PTFE sheet toadhere to other material, a heat-meltabletetrafluoroethylene/perfluoro(alkyl vinyl ether) copolymer (PFA) film isinterposed between the heat resistant woven fabric and the PTFE sheetand then heating is carried out for thermo-fusing at a temperature ofnot less than a melting point of PTFE. At the time of heating forthermo-fusing, PTFE is also melted unavoidably, and as a result,lowering of crystallinity, namely lowering of a specific weight arises.On the other hand, the higher the cystallinity (higher specific weight)of PTFE is, the lower the permeation of a chemical solution through PTFEis. Therefore it is better to avoid the heating which causes lowering ofthe crystallinity of PTFE. However if the heating is stopped in a stateof the crystallinity of PTFE remaining to a certain extent, a requiredadhesive strength cannot be obtained even if PFA is interposed.

In the conventional backing sheet, unintentionally a sheet of PTFEhaving a lowered crystallinity and a small specific weight was used as astarting material, or even if PTFE having a high crystallinity and ahigh specific weight was used as a starting material, lowering ofcrystallinity of the PTFE sheet by heating was obliged to be accepted.

Anyway, there was neither a backing sheet nor a laminated article ofPTFE having an average specific weight of not less than 2.175 and anadhesive strength of practicable level.

DISCLOSURE OF INVENTION

The first object of the present invention is to provide the PTFElaminated article being excellent in a low permeability to a chemicalsolution.

Also it is another object of the present invention to provide thebacking sheet having enough adhesive strength and a reduced permeabilityto a chemical solution.

Further an object of the present invention is to provide a method ofproducing the PTFE laminated article having a reduced permeability to achemical solution, in which a heating time can be shortened andproduction steps can be reduced.

Those objects can be achieved by a laminated article having a layeredstructure comprising a PTFE sheet having an average specific weight ofnot less than 2.175 and a heat-meltable resin layer or a layeredstructure obtained by laminating a PTFE sheet having an average specificweight of not less than 2.175 and a heat resistant woven fabric byinterposing a heat-meltable resin layer therebetween.

The obtained laminated article has an adhesive strength of not less than2 kgf/cm in a peel strength between the PTFE sheet and the heat-meltableresin layer, which is a strength equal to or more than that of aconventional laminated article comprising a PTFE sheet having an averagespecific weight of less than 2.175.

Hereupon an average specific weight means a specific weight of a wholePTFE sheet including a portion (layer) thereof having a lowered specificweight which arises as a result of heating.

When the laminated article of the present invention is used, forexample, as a backing sheet, in order to make a free surface of theoutermost PTFE sheet smooth, it is preferable to heat-treat the freesurface of the PTFE sheet at a temperature of not less than a meltingpoint of PTFE. In this surface smoothing treatment, it is preferablethat the sheet of PTFE is a sheet of PTFE modified with perfluoro(alkylvinyl ether) (PAVE).

The laminated article of the present invention can be produced bythermo-fusing by heating of a PTFE sheet, preferably a PTFE sheet havinga specific weight of more than 2.175 and a heat-meltable resin film orsheet while interposing between them a layer of un-sintered PTFE havinga melt energy of not more than 65 J/g, for example, a layer of fineparticles of PTFE homopolymer or PTFE modified with hexafluoropropylene(HFP) and/or perfluoro(alkyl vinyl ether) (PAVE).

The layer of PTFE fine particles can be applied in the form ofdispersion obtained by emulsion polymerization or in the form of filmobtained by drying the dispersion into a powder and then forming thepowder into a film.

Also the laminated article can be produced by heating for thermo-fusingof the PTFE sheet and heat-meltable resin film without interposing thelayer of PTFE fine particles. In that case, in order to make a specificweight of the PTFE sheet high, it is necessary to prevent a rapidlowering of temperature after the heating for thermo-fusing. By takingthis measure for preventing a rapid lowering of temperature,crystallization of molten PTFE advances sufficiently and the specificweight of PTFE can be made higher.

In order to prevent a rapid temperature lowering, for example, it isproper to hold a temperature within a range of less than a melting pointof PTFE and not less than 300° C. A holding time may be enough as far ascrystallization of molten PTFE advances sufficiently, and a timeinterval of 5 to 20 minutes can be adopted.

This measure for preventing a rapid temperature lowering can be takenalso in the case of interposing the layer of PTFE fine particles.

It is preferable that the heating is carried out from the side of heatresistant woven fabric, namely from the side of heat-meltable resin filmor sheet, and that the heating is stopped at the time when an un-meltinglayer remains in the PTFE sheet.

Particularly in the case of using as a backing sheet, it is preferablethat a surface of PTFE sheet opposite to the laminated surface, namely asurface contacting a solution is heat-treated at a temperature of notless than a melting point of PTFE to make the surface smooth. Theheat-treating may be carried out at any timing before or after theheating for thermo-fusing of the laminated article or at the same timeas the thermo-fusing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic cross-sectional view for explaining oneembodiment of the method for producing the PTFE laminated article of thepresent invention.

FIG. 2 is a diagrammatic cross-sectional view for explaining anotherembodiment of the method for producing the PTFE laminated article of thepresent invention.

BEST MODE FOR CARRYING OUT THE INVENTION

As mentioned above, the PTFE laminated article of the present inventioncan be produced by thermo-fusing by heating of a PTFE sheet and aheat-meltable resin film or sheet after interposing between them a layerof fine particles of un-sintered PTFE having a melt energy of not morethan 65 J/g (hereinafter referred to as “un-sintered PTFE” unlessotherwise noted) (First production method). The steps of the firstproduction method are explained below in detain, but the presentinvention is not limited to them.

At first a dispersion of un-sintered PTFE is applied on a PTFE sheet ofabout 1 mm to about 4 mm thick, followed by drying to form a layer offine particles of un-sintered PTFE on the PTFE sheet. A particle size offine particles of un-sintered PTFE is from about 0.1 μm to about 5 μm,preferably 0.1 to 0.5 μm from the viewpoint of a small melt energy andan increase in thermal conductivity. A concentration of the un-sinteredPTFE dispersion is from about 30% by weight (hereinafter referred to as“%”) to about 70%, preferably from about 30% to about 65% from theviewpoint of stabilizing the dispersion and enhancing a coatability. Acoating amount is from about 10 g/m² to about 160 g/m², preferably fromabout 15 g/m² to about 100 g/m² in a dry weight. When the coating amountis too small, non-uniform coating arises easily, and when too much,releasing of fine particles easily arises. The both cases are notpreferred.

A coating method is not limited particularly. For example, a spraycoating method, brush coating method and the like can be employed. Fordrying, air drying or forced drying by heating may be employed.

Instead of the un-sintered PTFE dispersion, a dispersion of un-moltentetrafluoroethylene/perfluoro(alkyl vinyl ether) copolymer (PFA) may beused, or an un-sintered PTFE film may be applied. The un-sintered PTFEfilm can be produced, for example, by coagulating a PTFE dispersion toform into a fine powder and then rolling the powder.

Then the heat-meltable resin film or sheet is overlaid on the layer(film) of un-sintered PTFE fine particles on the PTFE sheet, and as casedemands, after a heat resistant woven fabric is overlaid thereon,heating is carried out from the side of heat-meltable resin. In thepresent invention, this heating is important.

A heating temperature is not less than a melting point of PTFE (about327° C. to about 345° C.), preferably about 360° C. to about 390° C.,and it is important to stop the heating in a state that an un-moltenportion (layer) remains in the PTFE sheet at the mentioned temperature.Namely an amount of heat to be applied may be one in which crystalsremain in the PTFE sheet. If the crystals are molten completely, aspecific weight is greatly lowered below 2.175, and permeability to achemical solution is increased.

The heating time varies depending on a heating temperature, a thicknessof the PTFE sheet, kind and thickness of heat-meltable resin, athickness of heat resistant woven fabric, and the like, and may beselected experimentally or calculated from a degree of crystallization.For example, the heating time may be from 3 to 5 minutes in the casewhere a specific weight, thickness and heating temperature of the PTFEsheet are 2.189, 3 mm and 380° C., respectively.

Whether or not the un-molten portion remains partly can be recognized byseeing a cut surface of the PTFE sheet. Namely the PTFE sheet having ahigh crystallinity is white and opaque before heating, and becomestransparent when crystals are molten. Accordingly on the cut surface,there is a transparent portion (layer) at the side of heat-meltableresin and a white opaque layer remains at the non-heating side.

The method of heating for thermo-fusing may be such that, as shown inFIG. 1, a PTFE sheet 1, a layer 2 of un-sintered PTFE fine particles, aheat-meltable resin film or sheet 3 and as case demands, a heatresistant woven fabric 4 are arranged in that order, and a heating plate5 and a supporting plate 6 are arranged at the side of heat resistantwoven fabric 4 and PTFE sheet 1, respectively and then pressing with theheating plate 5 and supporting plate 6 and heating of the heating plate5 to a temperature of not less than a melting point of PTFE are carriedout.

Also as shown in FIG. 2, the laminated article may be producedcontinuously by winding a layered article comprising a PTFE sheet 1, alayer 2 of un-sintered PTFE fine particles, a heat-meltable resin filmor sheet 3 and as case demands, a heat resistant woven fabric 4, in thatorder on a heating roll 7 heated to a temperature of not less than amelting point of PTFE so that the heat resistant woven fabric 4 contactsa surface of the heating roll 7 and then heating for thermo-fusing whilepressing with a press roll 8. In FIG. 2, numeral 10 represents acrystallization zone, and numeral 11 represents a heater.

When the un-sintered PTFE dispersion is used, the layer 2 of un-sinteredPTFE fine particles may be formed by applying the un-sintered PTFEdispersion on the PTFE sheet 1 and then drying, as mentioned above, orby applying the un-sintered PTFE dispersion on the heat-meltable resinfilm or sheet 3 and then drying.

A pressure for pressing may be selected in a range of from about 0.1 MPato about 0.15 MPa. It is preferable from the viewpoint of makingcrystallinity high that after the heating, releasing of pressure andslow cooling to room temperature are carried out. The supporting plate 6and press roll 8 basically need not be heated, but from a point ofeliminating distortion of the laminated article, may be heated to atemperature of less than a melting point of PTFE, preferably to atemperature lower by 15° to 35° C. than the melting point of PTFE.

Then each material is explained below.

The PTFE sheet to be used in the present invention need to have a highcrystallinity, namely a specific weight exceeding 2.175, particularlynot less than 2.178, preferably 2.178 to 2.210. If the specific weightis small, permeability to a chemical solution increases and the objectsof the present invention cannot be achieved. The PTFE sheet having sucha high crystallinity can be obtained by a method described, for example,in the specification of PCT/JP98/01116, namely by producing a sinteredarticle through a rotation sintering method in which a PTFE moldedarticle obtained by compression-molding a PTFE powder is sintered whilebeing rotated, and then cutting the obtained sintered article to forminto a sheet. On the other hand, since a sheet obtained by cutting aPTFE sintered article produced by conventional method is waved greatly,the PTFE sheet must be made flat previously by heating in order to makea laminated article, and for that reason, a crystallinity of PTFE islowered before lamination. However in the rotation sintering method, aPTFE sheet being uniform and having a high crystallinity can beobtained, and yet a sheet obtained by cutting is flat and laminationwith other material is easy. Further flattening treatment by heating isnot necessary and there is no fear of lowering of crystallinity beforethe lamination.

The PTFE powder as a starting material is a tetrafluoroethylene (TFE)homopolymer or PTFE modified with other fluoromonomer. In the modifiedPTFE, a mole ratio of TFE and other fluoromonomer may be 95:5 to99.999:0.001. Examples of the modified PTFE are, for instance,PAVE-modified PTFE modified with perfluoro(alkyl vinyl ether) (PAVE),HFP-modified PTFE modified with hexafluoropropylene (HFP), and the like.Perfluoro(alkyl vinyl ether) (PAVE) may be a compound represented by theformula (I):CF₂=CF−OR_(f)  (I)wherein R_(f) is an organic group which has carbon atom and fluorineatom essentially, does not have hydrogen atom and may have oxygen atom.

The R_(f) group of perfluoro(alkyl vinyl ether) (I) may be aperfluoroalkyl group having 1 to 10 carbon atoms, aperfluoro(alkoxyalkyl) group having 4 to 9 carbon atoms, a grouprepresented by the formula (II):

wherein m is 0 or an integer of 1 to 4, or a group represented by theformula (III):

wherein n is 0 or an integer of 1 to 4.

Among them, PAVE-modified PTFE is preferred because it has a largecrystallinity and a specific weight exceeding 2.175 and is small inpermeability to a chemical solution.

A thickness of the PTFE sheet varies depending on intended applicationand is usually from 1 to 4 mm, and is from about 2 mm to about 4 mm whenthe sheet is used as a backing sheet.

The un-sintered PTFE dispersion for forming a layer of un-sintered PTFEfine particles can be obtained by emulsion-polymerizing TFE solely or inthe presence of a small amount of PAVE and/or HFP. It is preferable thata melt energy is not more than 65 J/g, particularly 30 to 50 J/g fromthe viewpoint of enhancing thermal conductivity. From the viewpoint ofexcellent thermo-fusing property, PAVE-modified PTFE is preferred.

The heat-meltable resin film or sheet may be one which is capable ofthermo-fusing with the PTFE sheet. Examples thereof are olefin resins;aromatic resins such as PPS, PES and PEEK; heat-meltablefluorine-containing resins such as TFE/PAVE copolymer (PFA) andTFE/hexafluoropropylene copolymer (FEP), and the like which have amelting point approximate to that of PTFE. Among them, from theviewpoint of having properties analogous to those of PTFE and havinggood adhesion to PTFE, preferred are heat-meltable fluorine-containingresins, and particularly for a backing sheet, PFA, FEP, and the like arepreferred.

The heat-meltable resin is usually used as an adhesive layer forallowing a PTFE sheet to adhere to other material (for example, heatresistant woven fabric). A laminated article produced by allowing a PTFEsheet to adhere to a heat-meltable resin sheet or a functional laminatedarticle produced by laminating a heat-meltable resin film to a PTFEsheet may be used. Therefore a thickness thereof may be optionallyselected depending on purpose. For example, when the heat-meltable resinis used as an adhesive layer of a backing sheet, a thickness thereof maybe from about 10 μm to about 300 μm.

Then explained below is a method (Second production method) forobtaining a laminated article by making a pre-laminated article bythermo-fusing a PTFE sheet and a heat-meltable resin film directly or byinterposing a layer of PTFE fine particles therebetween and thenallowing the pre-laminated article to stand in a state of not causing arapid temperature lowering to accelerate crystallization of molten PTFE.

The thermo-fusing treatment of the PTFE sheet and heat-meltable resinfilm may be the same as in the first production method, but a heatingtime may be slightly longer. The second production method has a featureof sufficiently carrying out re-crystallization of molten PTFE in thePTFE sheet melted by thermo-fusing treatment. The sufficientre-crystallization of PTFE can restore the PTFE sheet having a highspecific weight. Namely if the molten PTFE is cooled rapidly or allowedto stand as it is, crystallization is not advanced, and also generatedcrystals are incomplete ones. As a result a specific weight is notincreased.

Therefore in the second production method, rapid lowering oftemperature, which results in lowering of a specific weight, isprevented. For preventing rapid temperature lowering, there is a methodof allowing the molten PTFE to stand at a temperature of lower than amelting point (from about 327° C. to about 345° C.) of PTFE and not lessthan 300° C. for a period of time for advancing crystallizationsufficiently. This residence time varies depending on a temperature atthermo-fusing, a thickness of PTFE sheet, a specific weight of startingPTFE sheet, etc., and may be selected in a range of from 5 to 20minutes, preferably 5 to 10 minutes. For example, when a 3 mm thick PTFEsheet having a specific weight of 2.189 is thermo-fused at 380° C., itis possible to prevent the specific weight of PTFE sheet from loweringto less than 2.175 by holding the sheet at a temperature of 300° to 310°C. for 7 to 9 minutes.

As concrete means for preventing such a rapid temperature lowering, asshown in FIG. 2, there is a method of providing crystallization zonesrepresented by numeral 10 downstream of the heating roll 7 at both sidesof the PTFE sheet 1 and heat resistant woven fabric 4 and carrying out atemperature control of the thermo-fused PTFE sheet 1 with heaters 11 inthe crystallization zones 10 lest a rapid temperature lowering shouldarise. Also the crystallization zone may be designed so as to be in theform of crystallization chamber covering the whole laminated articlethough it is not illustrated.

The temperature control in the crystallization zone may be carried outby maintaining a constant temperature, continuously lowering thetemperature or lowering the temperature by stages (for example, 350° C.zone, 330° C. zone and then 310° C. zone).

When heating for thermo-fusing from the side of heat-meltable resin film(from the side of heat resistant woven fabric), means for preventing arapid temperature lowering may be provided at the side of heat-meltableresin film.

The laminated article of the present invention basically may have atwo-layered structure comprising a PTFE sheet and a heat-meltable resinfilm or sheet. Also in case of a composite laminated article produced bycombining other material by using a heat-meltable resin as an adhesivelayer as mentioned above, an effect can be exhibited especially.

In the case of applications making the best use of heat resistance ofPTFE, for example, in the case of a backing sheet, examples of the othermaterial are heat resistant woven fabrics, for example, woven fabrics ofglass, carbon, polyamideimide, boron nitride, and the like.

The laminated article of the present invention has a low permeability toa chemical solution and is useful as a backing sheet for lining ofvarious storing and transporting vessels, tank, pipe line, and the like.In addition, the laminated article is suitable for mold releasing andsliding applications by making the best use of non-sticking property andfriction property of PTFE.

In case of use for production of semi-conductors in which a very highcleanliness is required, it is not enough only to prevent a chemicalsolution from penetrating and oozing out, and it is necessary to protectthe chemical solution itself from contamination. For example, in casewhere the PTFE laminated article of the present invention (a surfacethereof contacting a solution is a PTFE sheet) is formed on an innersurface of a vessel for chemical solution, if a surface of the PTFEsheet obtained by cutting is rough, even if the inside of the vessel ismerely washed, a stain, particularly fine particles (calledmicroparticles) remain and contaminate the chemical solution. Thereforein the present invention, it is preferable that before laminating, whenlaminating or after laminating, a surface of the PTFE sheet whichcontacts a chemical solution is heat-treated at a temperature of notless than a melting point of PTFE, for example at 340° to 390° C.

This surface smoothing treatment is carried out by heating at atemperature of not less than a melting point of PTFE for a short periodof time under no load or under a light load. However attention must bepaid so that finally an average specific weight of the PTFE sheet in thelaminated article should not lower below 2.175. The surface may beheated by means of a burner or by increasing a temperature of thesupporting plate of a heat press (for example, FIG. 1) used forlamination to not less than a melting point of PTFE for a short periodof time. Further a method of passing the PTFE sheet on the heating rollfor a short period of time can be employed. A heating time is usuallyfrom 6 to 60 seconds in case where a heating temperature is 380° C. Bythe smoothing treatment, a PTFE sheet having a surface roughness (Ra) ofnot more than 350 nm, usually 100 to 250 nm can be obtained.

Then preferred embodiments of the laminated article of the presentinvention is explained below, but the present invention is not limitedto them.

-   (1) PTFE sheet: PAVE-modified PTFE. Average specific weight is    larger than 2.175.    -   Un-sintered PTFE fine particle layer: Coating of PAVE-modified        PTFE dispersion        -   Average particle size: 0.1 to 0.5 μm        -   Coating amount (dry): 15 to 100 g/m²        -   Melt energy: not more than 50 J/g    -   Heat-meltable resin film: PFA film (melting point: 310° C.)        -   Thickness: 10 to 300 μm    -   Other material: Woven fabrics of heat resistant glass, carbon,        polyamideimide and boron nitride    -   Other treatment: Surface smoothing treatment    -   Application: Backing sheet for lining of vessel, tank, pipe,        etc. for storing and transporting a chemical solution.        Particularly a vessel for transporting and storing a chemical        solution in production of semi-conductors.    -   Effect: Permeability to a chemical solution is low and an amount        of sticking microparticles is small since a surface is smooth.-   (2) PTFE sheet: PAVE-modified PTFE. Average specific weight is from    2.178 to 2.210.    -   Un-sintered PTFE fine particle layer: Film of PAVE-modified PTFE        fine particles        -   Average particle size: 0.1 to 0.5 μm        -   Coating thickness: 10 to 100 μm        -   Melt energy: not more than 50 J/g    -   Heat-meltable resin film: PFA film (melting point: 310° C.)        -   Thickness: 10 to 300 μm    -   Other material: Woven fabrics of heat resistant glass, carbon,        polyamideimide and boron nitride    -   Other treatment: Surface smoothing treatment    -   Application: Backing sheet for lining of vessel, tank, pipe,        etc. for storing and transporting a chemical solution.        Particularly a vessel for transporting and storing a chemical        solution in production of semi-conductors.    -   Effect: Permeability to a chemical solution is low and an amount        of sticking microparticles is small since a surface is smooth.-   (3) PTFE sheet: PAVE-modified PTFE. Average specific weight is from    2.178 to 2.210.    -   Un-sintered PTFE fine particle layer: Coating of PAVE-modified        PTFE dispersion        -   Average particle size: 0.1 to 0.5 μm        -   Coating amount (dry): 15 to 100 g/m²        -   Melt energy: not more than 50 J/g    -   Heat-meltable resin film: PFA film (melting point: 310° C.)        -   Thickness: 10 to 300 μm    -   Other material: Woven fabrics of heat resistant glass, carbon,        polyamideimide and boron nitride    -   Other treatment: Surface smoothing treatment    -   Application: Backing sheet for lining of vessel, tank, pipe,        etc. for storing and transporting a chemical solution.        Particularly a vessel for transporting and storing a chemical        solution in production of semi-conductors.    -   Effect: Permeability to a chemical solution is low and an amount        of sticking microparticles is small since a surface is smooth.-   (4) PTFE sheet: Un-modified PTFE. Average specific weight is from    2.178 to 2.210.    -   Un-sintered PTFE fine particle layer: Coating of PAVE-modified        PTFE dispersion        -   Average particle size: 0.1 to 0.5 μm        -   Coating amount (dry): 15 to 100 g/m²        -   Melt energy: not more than 50 J/g    -   Heat-meltable resin film: PFA film (melting point: 310° C.)        -   Thickness: 10 to 300 μm    -   Other material: Woven fabrics of heat resistant glass, carbon,        polyamideimide and boron nitride    -   Application: Backing sheet for lining of vessel, tank, pipe,        etc. for storing and transporting a chemical solution    -   Effect: Permeability to a chemical solution is low.-   (5) PTFE sheet: Un-modified PTFE. Average specific weight is from    2.178 to 2.210.    -   Un-sintered PTFE fine particle layer: Film of PAVE-modified PTFE        fine particles        -   Average particle size: 0.1 to 0.5 μm        -   Coating thickness: 10 to 100 μm        -   Melt energy: not more than 50 J/g    -   Heat-meltable resin film: PFA film (melting point: 310° C.)        -   Thickness: 10 to 300 μm    -   Other material: Woven fabrics of heat resistant glass, carbon,        polyamideimide and boron nitride    -   Application: Backing sheet for lining of vessel, tank, pipe,        etc. for storing and transporting a chemical solution    -   Effect: Permeability to a chemical solution is low.-   (6) PTFE sheet: PAVE-modified PTFE. Average specific weight is from    2.178 to 2.210.    -   Un-sintered PTFE fine particle layer: Coating of unmodified PTFE        dispersion        -   Average particle size: 0.1 to 0.5 μm        -   Coating amount (dry): 15 to 100 g/m²        -   Melt energy: not more than 50 J/g    -   Heat-meltable resin film: PFA film (melting point: 310° C.)        -   Thickness: 10 to 300 μm    -   Other material: Woven fabrics of heat resistant glass, carbon,        polyamideimide and boron nitride    -   Other treatment: Surface smoothing treatment    -   Application: Backing sheet for lining of vessel, tank, pipe,        etc. for storing and transporting a chemical solution.        Particularly a vessel for transporting and storing a chemical        solution in production of semi-conductors.    -   Effect: Permeability to a chemical solution is low and an amount        of sticking microparticles is small since a surface is smooth.-   (7) PTFE sheet: PAVE-modified PTFE. Average specific weight is from    2.178 to 2.210.    -   Un-sintered PTFE fine particle layer: Film of un-modified PTFE        fine particles        -   Average particle size: 0.1 to 0.5 μm        -   Coating thickness: 10 to 100 μm        -   Melt energy: not more than 50 J/g    -   Heat-meltable resin film: PFA film (melting point: 310° C.)        -   Thickness: 10 to 300 μm    -   Other material: Woven fabrics of heat resistant glass, carbon,        polyamideimide and boron nitride    -   Other treatment: Surface smoothing treatment    -   Application: Backing sheet for lining of vessel, tank, pipe,        etc. for storing and transporting a chemical solution.        Particularly a vessel for transporting and storing a chemical        solution in production of semi-conductors.    -   Effect: Permeability to a chemical solution is low and an amount        of sticking microparticles is small since a surface is smooth.-   (8) PTFE sheet: Un-modified PTFE. Average specific weight is from    2.178 to 2.210.    -   Un-sintered PTFE fine particle layer: Coating of unmodified PTFE        dispersion        -   Average particle size: 0.1 to 0.5 μm        -   Coating amount (dry): 15 to 100 g/m²        -   Melt energy: not more than 50 J/g    -   Heat-meltable resin film: PFA film (melting point: 310° C.)        -   Thickness: 10 to 300 μm    -   Other material: Woven fabrics of heat resistant glass, carbon,        polyamideimide and boron nitride    -   Application: Backing sheet for lining of vessel, tank, pipe,        etc. for storing and transporting a chemical solution    -   Effect: Permeability to a chemical solution is low.-   (9) PTFE sheet: Un-modified PTFE. Average specific weight is from    2.178 to 2.210.    -   Un-sintered PTFE fine particle layer: Film of un-modified PTFE        fine particles        -   Average particle size: 0.1 to 0.5 μm        -   Coating thickness: 10 to 100 μm        -   Melt energy: not more than 50 J/g    -   Heat-meltable resin film: PFA film (melting point: 310° C.)        -   Thickness: 10 to 300 μm    -   Other material: Woven fabrics of heat resistant glass, carbon,        polyamideimide and boron nitride    -   Application: Backing sheet for lining of vessel, tank, pipe,        etc. for storing and transporting a chemical solution    -   Effect: Permeability to a chemical solution is low.-   (10) PTFE sheet: PAVE-modified PTFE. Average specific weight is from    2.178 to 2.210.    -   Un-molten PFA fine particle layer: Coating of PFA dispersion        -   Average particle size: 0.1 to 0.5 μm        -   Coating amount (dry): 15 to 100 g/m²        -   Melt energy: not more than 50 J/g    -   Heat-meltable resin film: PFA film (melting point: 310° C.)        -   Thickness: 10 to 300 μm    -   Other material: Woven fabrics of heat resistant glass, carbon,        polyamideimide and boron nitride    -   Other treatment: Surface smoothing treatment    -   Application: Backing sheet for lining of vessel, tank, pipe,        etc. for storing and transporting a chemical solution.        Particularly a vessel for transporting and storing a chemical        solution in production of semi-conductors.    -   Effect: Permeability to a chemical solution is low and an amount        of sticking microparticles is small since a surface is smooth.-   (11) PTFE sheet: Un-modified PTFE. Average specific weight is from    2.178 to 2.210.    -   Un-molten PFA fine particle layer: Coating of PFA dispersion        -   Average particle size: 0.1 to 0.5 μm        -   Coating amount (dry): 15 to 100 g/m²        -   Melt energy: not more than 50 J/g    -   Heat-meltable resin film: PFA film (melting point: 310° C.)        -   Thickness: 10 to 300 μm    -   Other material: Woven fabrics of heat resistant glass, carbon,        polyamideimide and boron nitride    -   Application: Backing sheet for lining of vessel, tank, pipe,        etc. for storing and transporting a chemical solution    -   Effect: Permeability to a chemical solution is low.-   (12) PTFE sheet: PAVE-modified PTFE. Average specific weight is    larger than 2.175.    -   Heat-meltable resin film: PFA film (melting point: 310° C.)        -   Thickness: 10 to 300 μm    -   Other material: Woven fabrics of heat resistant glass, carbon,        polyamideimide and boron nitride    -   Other treatment: Treatment for preventing a rapid temperature        lowering after thermo-fusing    -   Application: Backing sheet for lining of vessel, tank, pipe,        etc. for storing and transporting a chemical solution.        Particularly a vessel for transporting and storing a chemical        solution in production of semi-conductors.    -   Effect: Permeability to a chemical solution is low and an amount        of sticking microparticles is small since a surface is smooth.-   (13) PTFE sheet: PAVE-modified PTFE. Average specific weight is from    2.178 to 2.210.    -   Heat-meltable resin film: PFA film (melting point: 310° C.)        -   Thickness: 10 to 300 μm    -   Other material: Woven fabrics of heat resistant glass, carbon,        polyamideimide and boron nitride    -   Other treatment: To maintain at a temperature of from 300° C. to        a temperature less than a melting point of PTFE for 5 to 20        minutes after thermo-fusing    -   Application: Backing sheet for lining of vessel, tank, pipe,        etc. for storing and transporting a chemical solution.        Particularly a vessel for transporting and storing a chemical        solution in production of semi-conductors.    -   Effect: Permeability to a chemical solution is low and an amount        of sticking microparticles is small since a surface is smooth.

Then the present invention is explained by means of examples, but is notlimited to them.

EXAMPLE 1

A PAVE-modified PTFE dispersion (melt energy: 40 J/g, average particlesize: 0.3 μm, concentration: 65%) prepared by emulsion polymerizationwas coated by brush coating on one surface of a PTFE sheet(PAVE-modified PTFE sheet having a specific weight of 2.191, width: 200mm, length: 200 mm, thickness: 3 mm) obtained by sintering by a rotationsintering method (method described in Example 2 of PCT/JP98/01116) andthen cutting so that a coating amount of PTFE fine particles wouldbecome 30 g/m² after drying. Then the dispersion was dried. Subsequentlya PFA film (width: 200 mm, length: 200 mm, thickness: 0.25 mm) wasoverlaid on the PTFE fine particle layer and further a heat resistantglass cloth (width: 200 mm, length: 200 mm, thickness: 3 mm) was placedthereon to give a layered article. The layered article was put betweenthe heating plate 5 and the supporting plate 6 shown in FIG. 1 andheated under the conditions that a temperature of the heating plate was380° C., a temperature of the supporting plate was 300° C. and anapplying pressure was 1 kgf/cm². Four minutes after, the pressure on theheating plate was released and the layered article was slowly cooleddown to room temperature over 2 to 3 minutes to give a laminated article(backing sheet) of the present invention.

A sample of 30 mm wide×150 mm long was obtained from the laminatedarticle by cutting it. An adhesive strength (peel strength) between thePTFE sheet and the PFA film of the sample which was measured accordingto JIS K 6772-9-5 was 3.0 kgf/cm. An average specific weight of thewhole PTFE sheet was 2.184. When the cut surface of the PTFE sheet wasobserved, it was recognized that about 30% from the side of PFA film wastransparent due to melting.

Further the obtained laminated article was heat-treated at 360° C. forone minute by a heat press to make a surface of the PTFE sheet smooth,but an average specific weight of the PTFE sheet was maintained high at2.183.

EXAMPLE 2

A PAVE-modified PTFE sheet (average specific weight: 2.191) was producedby sintering through conventional method and then cutting. Since thesheet was waved, it was subjected to smoothening treatment by heatingand pressing at 360° C. for five minutes and the average specific weightlowered to 2.178. A laminated article was produced in the same manner asin Example 1 by laying un-sintered PTFE fine particles, PFA film andheat resistant glass cloth and heating and pressing for two minutesunder the same conditions except that the PTFE sheet subjected tosmoothening treatment was used. An average specific weight of the PTFEsheet in the laminated article was 2.177, an adhesive strength (peelstrength) thereof was 3.2 kgf/cm and a proportion of transparent layerof the cut surface thereof was 10%.

Further the surface of the PTFE sheet was made smooth in the same manneras in Example 1, but the specific weight was 2.176 and was hardlylowered.

EXAMPLE 3

A laminated article was produced in the same manner as in Example 1 bylaying un-sintered PTFE fine particles, PFA film and heat resistantcloth and heating and pressing for two minutes under the same conditionsexcept that unmodified PTFE sheet obtained by the rotation sinteringmethod (average specific weight: 2.178) was used. An average specificweight of the PTFE sheet in the laminated article was 2.177, an adhesivestrength (peel strength) thereof was 3.2 kgf/cm and a proportion oftransparent layer of the cut surface thereof was 10%.

EXAMPLE 4

A laminated article was produced in the same manner as in Example 1except that un-sintered PTFE dispersion was not coated, a heating andpressing time was extended to 8 minutes and after held on a 305° C. heatpress for 8 minutes, a pressure was released and a layered article wasslowly cooled to room temperature over 2 to 3 minutes. An averagespecific weight of the PTFE sheet in the laminated article was 2.177 andan adhesive strength (peel strength) thereof was 3.2 kgf/cm.

COMPARATIVE EXAMPLE 1

A laminated article for comparison was produced by laying andheat-pressing in the same manner as in Example 1 except that anun-sintered PTFE dispersion was not coated. The PFA film could be peeledfrom the PTFE sheet easily by hand.

COMPARATIVE EXAMPLE 2

A laminated article was produced in the same manner as in ComparativeExample 1 except that a heating time was extended to 8 minutes. Anadhesive strength (peel strength) of the obtained laminated article was3.5 kgf/cm. However the PTFE sheet was transparent and an averagespecific weight was as small as 2.170.

INDUSTRIAL APPLICABILITY

According to the present invention, a PTFE laminated article having asufficient adhesive strength can be obtained while a specific weight ofa PTFE sheet can be maintained high, and a PTFE backing sheet having lowpermeability to a chemical solution can be provided.

1. A laminated article having a heat-fusing layered structure obtainedby laminating a polytetrafluoroethylene sheet having an average specificweight of not less than 2.175 and a heat resistant woven fabric byinterposing a heat-meltable resin layer therebetween, wherein saidpolytetrafluoroethylene sheet of said laminate has an average specificweight of not less than 2.175 and said polytetrafluoroethylene ispolytetrafluoroethylene modified with perfluoro(alkyl vinyl ether). 2.The laminated article of claim 1, wherein the laminated article is abacking sheet.
 3. The laminated article of claim 1, wherein an adhesivestrength between the polytetrafluoroethylene sheet and the heat-meltableresin layer is not less than 2 kgf/cm in a peel strength.
 4. Thelaminated article of claim 1, wherein said polytetrafluoroethylene sheetcomprises a high specific weight layer having a specific weightexceeding 2.175 and a low specific weight layer having a specific weightlower than that of the high specific weight layer.
 5. The laminatedarticle of claim 1, wherein said heat-meltable resin is a heat-meltablefluorine-containing resin.
 6. The laminated article of claim 5, whereinsaid heat-meltable fluorine-containing resin is atetrafluoroethylene/perfluoro(alkyl vinyl ether) copolymer.
 7. Alaminated article having a heat-fusing layered structure comprising apolytetrafluoroethylene sheet having an average specific weight of notless than 2.175 and a heat-meltable resin layer, wherein saidpolytetrafluoroethylene sheet of said laminate has an average specificweight of not less than 2.175 and said polytetrafluoroethylene ispolytetrafluoroethylene modified with perfluoro(alkyl vinyl ether), andwherein said polytetrafluoroethylene sheet comprises a high specificweight layer having a specific weight exceeding 2.175 and a low specificweight layer having a specific weight lower than that of the highspecific weight layer.